Air bag systems generally used in the past are of the type that inflates one air bag using a single inflator. In this type of air bag system, the change of acceleration of a vehicle is constantly monitored using an acceleration sensor mounted in a passenger compartment, and the resulting acceleration signal is processed by performing appropriate mathematical operations such as first integration or second integration; then, the result is compared with a predetermined threshold value and, if it exceeds the threshold value, an activation signal is issued to an inflator ignition circuit to activate the inflator and thus inflate the air bag.
Since this type of air bag system is designed, based on safety standards, to produce maximum performance in a frontal collision at 50 km/h, the air bag is inflated with designated characteristics if only the threshold value is exceeded, regardless of the severity of the collision or the position or posture of the vehicle occupant. Therefore, in a low-speed or medium-speed collision, the air bag is inflated with a inflating energy excessive for occupant protection, giving rise to the possibility that, if the occupant is seated close to the air bag, or in the case of a small sized occupant, the occupant may be injured by the inflation of the air bag.
As for the acceleration sensor used to determine whether to activate or not activate the inflator, there are two types according to the sensor mounting position: one is the integral type in which the sensor is assembled into the air bag module and mounted in the steering wheel, and the other is the separate type in which the sensor is on the driver's seat side in the passenger compartment. In the case of the integral type, the acceleration sensor detects the impact of the collision transmitted through the steering shaft, and in the case of the separate type, the acceleration sensor is mounted on a bracket attached to the vehicle body and detects the impact of the collision transmitted to the inside of the passenger compartment through the vehicle body; in either case, the decision whether to inflate or not inflate the air bag is made based on the change of acceleration detected by the acceleration sensor mounted within the passenger compartment that has a structure of high rigidity and is less subjected to deformation in the event of a collision.
Some vehicle types in which impacts at the front of the vehicle are difficult to transmit to the inside of the passenger compartment employ a system that uses an electronic acceleration sensor mounted inside the passenger compartment in combination with a mechanical sensor mounted in a crush zone, such as an engine compartment, in the forward part of the vehicle, but since the mechanical sensor, because of its characteristics, is only capable of making an ON/OFF decision and is used in conjunction with a collision discrimination system that uses the acceleration sensor mounted inside the passenger compartment, if a localized impact such as hammering is input to the mechanical sensor, an erroneous activation may result.
In recent years, there has been proposed a system generally known as the "smart air bag system" which employs a plurality of inflators and controls the mode of air bag inflation in an optimum manner by controlling the output level of the inflators in accordance with the type of collision and the condition of the occupants. To implement this system, an ignition decision with a timing earlier than the conventional ignition decision timing becomes necessary to perform computations for inflator output control, but such an early decision system has yet to be proposed.
The present invention has been devised to address the above problems, and an object of the invention is to provide a novel air bag activation control apparatus that can make the ignition decision earlier and more timely than previous systems, and that drastically reduces the possibility of erroneous activation by correctly discriminating impacts even in situations of improper use (hereinafter called the "abuse") such as hammering or rough road driving that could result in an erroneous activation if the discrimination were made replying only on the passenger compartment acceleration sensor.